This invention relates to a process for the hydrogenation of long chain unsaturated olefinic compounds, and more particularly, the hardening of animal and vegetable oils to produce salad oil, margarine, shortening and the like using Raney nickel alloy catalysts as hydrogenation promoters.
Raney nickel is a well-known hydrogenation catalyst which was described originally in U.S. Pat. No. 1,638,190 issued to Raney on May 10, 1927. Raney nickel is prepared by alloying nickel and aluminum and leaching out the aluminum with alkali to expose nickel as a finely divided porous solid in which form nickel is an effective hydrogenation catalyst.
Subsequently, improved nickel catalysts have been provided in the art by alloying various metallic constituents with the nickel and aluminum prior to the treatment with alkali. For example, in U.S. Pat. No. 2,948,687 issued to Hadley on Aug. 9, 1960, molybdenum is alloyed with nickel and aluminum and treated with alkali to provide a nickel-molybdenum alloy catalyst. The use of such catalysts either as finely divided powders or precipitated onto a support structure such as Al.sub.2 O.sub.3 is well known and such catalysts are widely used at the present time in a number of catalytic synthesis operations.
One industry in which such catalysts find use is the "hardening" of crude vegetable and animal oils such as soybean oil, corn oil, cod liver and whale blubber oil to produce commercial products for use in foods, lubricants, paints, fuels and the like. In the "hardening" process, the oil is hydrogenated to reduce the total unsaturated content as measured by the iodine value of the oil and consequently raise the melting point of the oil so treated. Of particular importance in many hydrogenation techniques is the removal of the triene unsaturation (linolenic acid), which is primarily responsible for problems of rancidity and off-taste of soybean oil and similar oils.
The modern hydrogenation process for edible products such as salad oil, margarine and vegetable shortening originated in research work conducted at the turn of this century. In this process, edible oils, such as cottonseed, soybean, and corn oil, are placed within a reaction vessel (commonly termed a "convertor") and brought into contact with hydrogen at elevated temperature and pressure in the presence of a small amount of metal hydrogenation catalyst. In modern units, the catalyst is usually present in small amounts which may range from 0.01 to about 0.5 percent by weight based upon the total weight of the oil being subjected to hydrogenation. With soybean oil, such hydrogenation techniques can produce commercially useful salad oil, margarine and shortening products having nominal unsaturation percentages and iodine values as shown in Table I. Whichever product is produced is primarily a function of the process conditions utilized.
TABLE I ______________________________________ "HARDENED" SOYBEAN OIL PRODUCT COMPSITIONS Palmitic and Lino- Lino- Stearic* Oleic* lenic* lenic* Iodine Acids Acid Acid Acid Value (C16:0/C18:0) (C18:1) (C18:2) (C18:3) (IV)** ______________________________________ "crude" oil 14.5 22.1 55 8.4 130-140 salad oil 21 39 37 3 110-118 margarine 29 37 31 2.5 95-102 shortening 33 40 25 2 80-90 ______________________________________ *Nominal percentages as reported in the literature. **As determined by the Wijs procedure.
Various types of hydrogenation catalysts are known for providing the reaction between hydrogen and the edible oil. For example, one commercial hydrogenation catalyst includes the metal nickel as the principal catalytic agent, but it also may have minor amounts of copper, alumina, or other materials. The metal hydrogenation catalysts are employed principally in a finely defined divided form and are prepared by a variety of methods. Commonly, the nickel metal is placed upon a finely divided, highly porous, inert refactors material, such as diatomaceous earth, or other highly siliceous material. The catalyst is suspended in the oil during the hydrogenation process as an oil-coated inert granular solid, which may adsorb soaps or other impurities often found in the crude oil.
After the hydrogenation reaction is completed to the desired degree, the reaction materials are removed from the convertor and passed through a filtration system to remove the inorganic solids from the hydrogenated edible oil product. Various inorganic materials may also be added at this time to the oil to enhance its filterability. Generally, pressurized filter press assemblies are used, in parallel flow arrangements comprising a plurality of filter elements made from paper, canvas or other types of filter media. These filter elements may be precoated with some type of diatomaceous earth or other filter aid to improve oil filterability. However, it is found that such filters do not always remove all of the inorganic solids and secondary "polishing" steps may also be required.
In the process as described above, it has been known for many years that Raney catalysts, in the form of granular powders, will selectively harden crude animal and vegetable oils. However, they are not widely utilized for this purpose since many Raney catalysts are quite sensitive to low levels of thiophene and other forms of sulfur contamination in the feedstock. Also, finely granulated Raney catalysts are difficult to handle, with many being sufficiently active to exhibit pyrophoric or self-ignition properties if exposed to air. Where supported catalysts are used, it is further found that poorer control over the total process is achieved dye to a small portion of the oil being absorbed for some period of time by the support. This results in a non-uniform contact time with the catalyst and consequent increased variability in the product output. Also, many supporting media exhibit some catalytic activity themselves so that additional negative effects in the compositional integrity of the final product may occur. Such effects may include excessive cis-trans conversion, double bond migration, and co-polymerization. All of these effects, even if occurring at low levels are undesirable from the standpoint of producing edible products.
Further, with such catalysts it is more difficult to control the exothermic temperature rise experienced during the reaction. Such an occurrence can result in local overheating and cracking of the carbonaceous feedstock to form coke, tars and other adhesive products on the surfaces quickly leading to a significant decrease in catalytic activity.
It is also known that continuous processing techniques can be used for oil processing. However, while many processes, such as that described by Coombes et al in U.S. Pat. No. 3,792,067 dated Feb. 12, 1974, have been proposed, there are presently few such systems now operating in the United States; with most of these more nearly resembling a serial plurality of batch reactors than a true unitary continuous operation of the type as commonly used, for example, in many petrochemical synthesis processes. Results reported for such systems, summarized in Table II, show relatively low flow rates and poor olefinic compositional percentages due to the low linolenic and linoleic conversion selectivities found with many of the catalysts used, as compared to the nominal values for commercially useful products shown in Table I. Further, there seem to be perceived difficulties in changing feedstock or end product, and in keeping the catalyst surfaces clean of deposits from the small quantities of soaps and other gels found in most commercially produced oils.
Recently, Gray has disclosed, in U.S. Pat. No. 4,240,895 dated Dec. 23, 1980, a Raney material comprised of a monolithic metallic mesh core having an integral Beta structured Raney nickel alloy exterior surface thereon which is used as an electrode to reduce hydrogen overvoltage.
TABLE II __________________________________________________________________________ PRIOR ART "CONTINUOUS" HYDROGENATION OF SOYBEAN OIL Space Composition (%) Study Temp. Pressure Velocity C16:0/ Isomerization # Catalyst (.degree.C.) (psig) (1/hr-kg) C18:0 C18:1 C18:2 C18:3 IV**** Index*** Selectivity __________________________________________________________________________ 1 Raney Ni* 145 30 0.8 24.7 39.4 32.4 3.5 95.3 0.48 1.about.1.5 on Al.sub.2 O.sub.3 2 Raney Ni* 190 127.5 0.4 18.7 39.5 37.9 3.9 106.8 0.78 1.5.about.2 on Al.sub.2 O.sub.3 3 Pd/Al.sub.2 O.sub.3 * 180 30 1.0 19.9 28.7 46 5.4 110 0.81 1.3 4 Cu/Chromite* 185 60 0.48 13.6 25.8 54.3 6.3 125 0.96 1.2 5 Ni:Al* 100 75 7.6 22.3 31.7 40.8 5.8 108 0.60 1.5.about.2 (50:50) 6 Pd:Al* 110 23 -- 17.1 24.8 50.3 7.8 119 0.55 1.1 (5.95) 7 Cu/Cr.sub.2 O.sub.3 ** 153 30 0.3 16.7 42.9 37.8 2.6 109 0.65 3 (78:20) 8 Cu/Cr.sub.2 O.sub.3 ** 120 30 0.25 16.1 46.4 36.7 0.8 106 .about.0.70 4 (78:20) "Crude" 14.5 22.1 55.0 8.4 130-140 Soybean Oil __________________________________________________________________________ *JAOCS 52, 282 (1975); fixedbed reactor was used. **JAOCS 59, 333 (1982); tricklebed reactor was used. ***Isomerization Index = .DELTA.% transisomer/.DELTA.IV. ****Iodine value.